Age hardening silicon-containing maraging steel



United States Patent 3,294,527 AGE HARDENING SILICON-CONTAININ G MARA-GING STEEL Stephen F loreen, Westfield, and Raymond Frank Decker, Fanwood, N.J.,as'signors to The International Nickel Company, Inc New York, N.Y., a corporation of Delaware No Drawing. Filed June 9, 1964, Ser. No. 373,872 4 Claims. (Cl. 75-123) The present invention relates to ferrous-base alloys and, more particularly, to ferrous-base alloys manifesting a combination of relatively high yield strength and good toughness characteristics of such magnitude that they are eminently suitable for use as steels for high strength structural members.

As is well known by those skilled in the art, a number of mild carbon and low alloy steels have been developed and/ or proposed for use as structural steels. Such steels in the quenched and tempered condition have yield strengths on the order of about up to 100,000 pounds per square inch (p.s.i.). While there are low alloy steels which afford yield strengths of over 200,000 p.s.i., they generally lack adequate toughness to be categorized as structural steels.

A number of alloying constituents have been advanced to achieve higher yield strengths together with good toughness in non-quenched structural steels. Rarely, however, has silicon been accorded much significance. Of course, the use of silicon in ferrous-base alloys has been more than common. For example, various cast irons contain substantial amounts of silicon to attain enhanced corrosion and/or scaling resistance upon exposure to corrosive or oxidizing media. In chilled cast iron, silicon plays a major role in controlling the degree or depth of chill. With regard to steels, silicon has long been used in achieving preferred grain orientation, thus finding considerable use in applications requiring good magnetic properties. However, instructural steels, including the carbon and low alloy varieties, silicon is normally utilized for 'deoxidizing and/or malleableizing purposes, i.e., it is used to provide killed steels and to achieve fine grain structures. Occasionally, it is used in amounts up to about 1% in structural steels for certain applications not requiring a high degree of toughness. At page 228 of the 8th Edition of the Metals Handbook (1961), it is indicated that silicon contents above about 0.3% adversely affects the ability of prior art steels to absorb impact energy. Notwithstanding the heretofore characteristic role of silicon upon the toughness of steels, it has now been found that quite a satisfactory level of toughness, including notch toughness, can be achieved together with high yield strengths in silicon-containing steels, provided the amount of silicon is controlled as will be described herein. In this connection, it is to be understood that achieving a good level of toughness in the absence of high yield strength, e.g., 150,000 p.s.i., or vice versa, does not satisfy the prerequisites of the invention. i

It has now been discovered that when special amounts of silicon are incorporated in substantially carbon-free, nickel-containing, ferrous-base alloys, and most advantageously alloys containing small but efiective amounts of titanium and/or aluminum, an age-ha'rdenable steel is provided which atfords, upon simple heat treatment, a

ice

. good combination of yield strength and toughness. The

alloys of the present invention are of the class known as the maraging steels which were developed and introduced to the art several years ago. These steels have not only opened up a new area of steel metallurgy and technology, but fundamentally differ from the carbon and low alloy steels in that, inter alia, upon heating (aging) in the martensitic condition, the maraging steels increase in hardness and strength as opposed to the softening response and decrease in strength attendant the carbon and low alloy steels. It should be mentioned that with the advent of the maraging steels, silicon has been known to be a hardener in such steels, but that in amounts of, say, 0.5% and above, it has resulted in serious loss of ductility and toughness in prior art alloys.

It is an object of the present invention to provide a novel, silicon-containing age-hardenable steel capable of providing high yield strengths combined with good toughness in the aged condition.

Other objects and advantages will become apparent from the following description.

Generally speaking, the present invention contemplates providing steels consisting essentially of (by weight) about 10% to about 20% nickel, from 1.75% silicon, e.g., 2%, to not more than 4% silicon, carbon in an amount up to not more than 0.03%, up to 5% chromium, with the sum of the nickel plus chromium plus silicon contents being from 12% to 23%, up to 1% titanium, up to 1% aluminum, with the sum of the titanium plus aluminum being not greater than 1%, up to about 0.75% manganese, and the balance essentially iron. To achieve an optimum combination of properties, including yield strength and toughness, the steels most advantageously contain about 16% to about 20% nickel, about 2.5% to about 3.5% silicon, up to 1% chromium, with the sum of the nickel plus silicon and chromium contents being about 18.5% to 23%, at least one element selected from the group consisting of titanium in an amount of about 0.05% to 0.15% and aluminum in an amount of'up to about 0.15 and most advantageously in an amount of about 0.05 to about 0.15%, about 0.01% to 0.03% carbon, manganese in an amount of up to 0.3% e.g., 0.02% to 0.1% manganese, and the balance essentially iron. When heat treated by aging, steels of the present invention exhibit yield strengths (0.2% otfset) of from about 150,000 to 225,000 p.s.i., together with a highly satisfactory magnitude of toughness as determined by each of the following: a tensile elongation of at least 10% inch bar, 1 inch gage length), a reduction of area of at least 50% /4 inch bar) and a notch tensile to tensile strength ratio of at least unity and most advantageously of at least 1.1 (bar having a 0.3 inch major diameter, a 0.212 inch root diameter and a notch acuity factor (K of 12).

The following auxiliary elements in a total aggregate of not more than 2% can be present: up to 2% cobalt, up to 2% molybdenum, up to 2% vanadium, up to 2% tungsten, up to 0.2% beryllium, up to 2% columbium, up to 2% tantalum and up to 2% copper.

In carrying the invention into practice, the nickel content is advantageously from 16% to 20%, e.g., 18%. Lower amounts of nickel result in lower strength levels whereas higher amounts promote retention of austenite upon cooling from solution annealing temperatures to about room temperature, i.e., the structure obtained on cooling may not be as completely martensitic as would otherwise be the case, thus, necessitating, for example, a sub-zero cooling treatment to effect as complete a transformation to martensite as possible.

The amount of silicon employed has been found to be most important. When the silicon content is less than about 1.75% the yield strength is adversely affected whereas with silicon contents above 4%, toughness is detrimentally impaired. For the best combination of properties, it has been found that the silicon content should be within the range of 2.5% to 3.5%, e.g., 2.5% to 3.3%.

With regard to the amount of titanium and aluminum in the steels, it is advantageous that at least 0.05%, e.g., 0.1%, and up to 0.15% of each of these elements be present. In such amounts these elements contribute to achieving optimum toughness in the steels. Up to 1% of each of these elements (not more than 1% in the aggregate) can be present in the steels but toughness is lowered at the higher levels. While up to 0.75% manganese can be present, it is most preferred that the amount there-of not exceed about 0.1%; otherwise the toughness of the steels is considerably reduced.

The amount of carbon should not exceed 0.03%. With carbon contents characteristic of various carbon and low alloy steels, toughness is radically impaired and higher strengths are not attained. The steels should be substantially devoid of elements such as sulfur, phosphorous, oxygen, nitrogen, lead, etc., as is consistent with commercial steelmaking practice. Such constituents merely serve to give rise to problems involving processing of the steels, e.g., workability, and result in a deterioration of properties.

A most advantageous range for steels contemplated by the present invention is as follows: about 17.5% to 18.5% nickel, e.g., 18% nickel, about 2.75% to 3.3% silicon, e.g., 3% silicon, about 0.07% to 0.12%, e.g., 0.1%, titanium, about 0.7 to 0.12%, e.g., 0.1%, aluminum, about 0.02% to 0.08% manganese, e.g., 0.50% manganese, about 0.006% to 0.025% carbon, e.g., 0.02% carinn, and the balance essentially iron.

In carrying the invention into practice, the steels should be prepared using materials of relatively high purity and/or selected scrap as the basic melting charge. Low carbon, ferro-silicon or silicon metal can be advantageously utilized to adjust the silicon content and the usual malleableizing and/or deoxidizin-g constituents, e.g., boron, zirconium, calcium, lithium, magnesium and the like, can be employed. The cast ingots obtained upon solidification of the melt should be rather thoroughly homogenized by soaking at temperatures of about 2200 F. to 2400 F. followed by hot working and, if desired, cold worked to desired size. Prior to aging, the steels should be solution annealed at temperatures of from about 1400 F. to about 1600 F., although temperatures up to 2000 F. can be employed to some advantage in achieving optimum toughness. While the annealing treatment is not mandatory, it provides greater assurance of obtaining reproducible properties. The steels are then cooled to achieve a martensitic condition, i.e., a transformation from austenite to martensite. It is important that a martensitic structure be obtained prior to aging in order to achieve the high strength levels characteristic of the steels. Thus, as used herein, the term martensite (or martensitic) refers to steels having a structure of martensite (or substantially martensitic) in both the solution annealed condition and in the aged condition. The transformation from austenite to martensite is normally accomplished by cooling through the M -M, transformation range to room temperature (from the annealing temperature). In some instances, and in order to attain as complete a transformation as possible, it may be desirable to cool the steels below room temperature, e.g., down to minus 100 F., as by, for example, refrigeration. To illustrate, should the total content of nickel plus silicon plus chromium exceed about 22%, sub-zero cooling would be beneficial. Cold working prior to aging can also be used to effect the completion of transformation to martensite.

The aging treatment should be conducted over the temperature range of about 700 F. to about 900 F. and preferably 750 F. to 850 F. for about 1 hour to 50 hours, e.g., 3 to 24 hours at 800 F. Aging temperatures appreciably above 900 F. should be avoided; otherwise retention of undesirable austenite can occur. In addition, overaging of the precipitate can occur and thus cause or induce an undesired age-hardening effect. Since aging temperatures of 700 F. require undesirably long aging times, it is preferable to use an aging temperature range of 750 F. to 850 F. Generally, shorter periods should be used at the higher aging temperature.

For the purpose of giving those skilled in the art a better understanding and/ or a better appreciation of the advantages of the invention, the following illustrative data are given:

A series of alloys having compositions given in Table I (Alloys A, B, C and D being outside the invention and Alloys 1 through 5 being within the invention) were melted in an air induction furnace, cast into billets, homogenized, hot forged and then hot rolled to 4-inch bars. Specimens were solution annealed at about 1500 F. for 1 hour, air cooled and then refrigerated at minus F. for about 16 hours to assure, as a precautionary measure, complete transformation to martensite. The steels were thereafter aged at 800 F. for either 3 or 24 hours.

TABLE I Percent Alloy No.

Ni Si C Al Ti Mn Fe 18.1 0.55 0. 000 0. 05 0.08 0. 05 Eat. Ess. 18.4 0. 96 0.015 n.a. n.a. 0.09 Bal. Ess. 16. 7 1. 87 0.010 0. 05 0. 08 0.04 Bal. Ess. 18.4 2. 02 0.018 n.a. n.a. 0.1 Bal. Ess. 17. 8 2. 58 0. 008 0. 05 0.07 0. 06 Bal. Ess. 18.3 3.18 0.010 n.a. n.a. 0.11 13211. Ess. 18. 5 3. 30 0. 006 0. 05 0. 11 0. l0 Bal. ESS. 18.1 4. 27 0.018 0.07 O. 09 0.10 Bal. Ess. 18.5 4. 75 0.35 n.a. n.a. 0.07 Bal. Ess.

Bal. Ess.:balanco of alloy was essentially iron. n.a.=not added.

The data in Table II illustrate the Rockwell C hardness behavior of steels within the invention as exemplified by Alloy No. 3 upon aging at the temperatures and for the periods of time given in Table II.

TABLE II Hardness, Rockwell C Time (hours) Temp. F

(N.T.S., K Si), and the ratio of notch tensile to ultimate tensile strength (N.T.S./-U.T.S.) are given in Table III, the alloys being aged at 800 F. for the periods set forth:

the invention, as those skilled in the art will readily understand. Such modifications and variations are considered TABLE III Aging Y.S., U.T.S., EL, R.A., N.'I.S., N.T.S/ Alloy Time, K s.i K s.i. percent percent K s.i. U.T.S

hours 1 1% inch gage length.

factor (KL) of 12.

3 Broke into pieces.

The data in Table III reflect the importance of the 25 to be within the purview and scope of the invention and silicon content of the steels and also illustrate the beneficial efiects of small amounts of titanium and aluminum. For example, Alloys A and B having silicon contents below that prescribed herein exhibited markedly low yield strengths. Increasing the silicon content of Alloy A (0.55%) by a factor of slightly less than two (0.96% of Alloy B) did not result in any significant improvement. However, with silicon contents within the invention, yield strengths of well above 150,000 p.s.i., e.g., 180,000 psi. and upwards of 220,000 p.s.i., were obtained together with good toughness. The resistance to concentrated stress of the steels within the invention was exceedingly good as illustrated by the notch tensile strengths and ratio of notch tensile strength to ultimate tensile strength. While the yield strength of Alloy C (an alloy outside the invention by virtue of a silicon content of 4.27%) was high, its toughness was markedly poor as evidenced by both the tensile elongation and reduction of area values and, particularly, by the markedly inferior notch toughness characteristics. Alloy D contained, in addition to 4.75% silicon, carbon in an amount of 0.35%, a factor which also contributed to the markedly inferior properties characteristic of this steel. As mentioned above herein, the carbon content of the steels should be maintained at very low levels, e.g., 0.02% carbon, and should not exceed 0.03%.

The present invention is particularly applicable to the production of high strength structural elements, assemblies and the like. The aged alloys of the invention can be used as sheet, plate, rods, bars, extrusion, etc. A particular advantage of the contemplated alloys is that they can be readily machined and/or otherwise shaped or formed while in the annealed condition and can thereafter be age hardened without the incurrence of detrimental distortion or dimensional change being induced by the age-hardening treatment. The necessity of utilizing further processing steps to obviate distortion and/ or dimensional change has been characteristic of prior art carbon and low alloy steels.

As will be readily understood by those skilled in the art, the term balance as used herein in referring to the iron content of the alloys does not preclude the presence of other elements, e.g., deoxidizing and cleansing elements, and impurities normally associated therewith in small amounts which do not adversely afiect the basic characteristics of the alloys.

Although the :present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of appended claims.

We claim:

1. A martensitic steel characterized in the aged condition by a combination of relatively high yield strength together with good toughness, said steel consisting essentially of about 16 %to 20% nickel, about 2.5% to 3.5% silicon, up to 1% chromium, the sum of the nickel plus silicon plus chromium contents being about 18.5% to 23 carbon in an amount up to not more than 0.03 at least one metal selected from the group consisting of titanium in an amount of about 0.05% to 0.15% and aluminum in an amount up to 0.15 manganese in an amount up to about 0.1%, and the balance essentially 1IO11.

2. A martensitic steel characterized in the aged condition by a combination of relatively high yield strength together with good toughness and having a composition within the ranges of about 10% to 20% nickel, from about 1.75% to not more than 4% silicon, carbon in an amount up to not more than 0.03%, up to 5% chromium with the sum of the nickel plus silicon plus chromium contents not exceeding about 23%, up to about 1% titanium, up to about 1% aluminum, with the sum of the titanium plus aluminum not exceeding 1%, up to 0.75% manganese, up to 2% cobalt, up to 2% molybdenum, up to 2% vanadium, up to 2% tungsten, up to 0.2% beryllium, up to 2% columbium, up to 2% tantalum, up to 2% copper, with the proviso that the total summation of cobalt, molybdenum, vanadium, tungsten, beryllium, columbium, tantalum and copper is not greater than 2%, and the balance essentially iron.

3. The alloy as set forth in claim 2 wherein the manganese content does not exceed 0.1%

4. A martensitic steel characterized in the aged condition by a combination of relatively high yield strength together with good toughness, said steel consisting essentially of about 17.5% to 18.5% nickel, about 2.75% to 3.3% silicon, 0.006% to 0.025% carbon, about 0.07% to 0.12% titanium, about 0.07% to 0.12 aluminum, 0.02% to 0.08% manganese, and the balance essentially iron.

References Cited by the Examiner UNITED STATES PATENTS 1,759,477 5/1930 Armstrong 123 2,048,164 7/1936 Pilling 148-142 2,060,765 11/1936 Welch 75123 X 2,602,736 7/ 1952 Sheridan 75123 DAVID L. RECK, Primary Examiner.

P. WEINSTEIN, Assistant Examiner. 

2. A MARTENSITIC STEEL CHARCTERIZED IN THE AGED CONDITION BY A COMBINATION OF RELATIVELY HEIGH YIELD STRENGTH TOGETHER WITH GOOD TOUGHNESS AND HAVING A COMPOSITION WITHIN THE RANGEE OF ABOUT 10% TO 20% NICKEL, FROM ABOUT 1.75% TO NOT MORE THAN 4% SILICON, CARBON IN AN AMOUNT UP TO NOT MORE THAN 0.03%, UP TO 5% CHROMIUM WITH THE SUM OF THE NICKEL PLUS SILICON PLUS CHROMIUM CONTENTS NOT EXCEEDING ABOUT 23%, UP TO ABOUT 1% TITANIUM, UP TO ABOUT 1% ALUMINUM, WITH THE SUM OF THE TITANIUM PLUS ALUMINUM EEEDING 1%, UP TO 0.75% MANGANESE, UP TO 2% COBALT, UP TO2% MOLYBDENUM,UP TO 2% VANADIUM UP TO 2% TUNGSTEN, UP TO 0.2% BERYLLIUM, UP TO 2% COLLUMBIUM UP TO 2% TANTALUM, UP TO 2% COPPER, WITH THE PROVISO THAT THE TOTAL SUMMATION OF COBALT, MOLYBDENUM, VANADIUM, TUNGSTEN, BERYLLIUM, COLUMBIUM, TANTALUM AND COPPER IS NOT GREATER THAN 2%, AND THE BALANCE ESSENTIALLY IRON. 